In an integrated voice and data network, network resources such as switching nodes and wideband transmission links are efficiently shared by voice and data for improved cost effectiveness. In the prior art, two schemes have been proposed for integrated multiplexing of voice and data on wideband transmission links. They are (1) the movable boundary scheme, which is described by K. Sriram et al., in the IEEE Journal on Selected Areas in Communications, Vol. SAC-1, No. 6, December 1983, pages 1124-1132, and (2) the burst switching scheme, which is described by E. F. Haselton, in IEEE Communications Magazine, September 1983, pages 13-19. Voice is digitized so as to facilitate integration with data. Speed detection is employed, and transmission bandwidth is only allocated to voice sources during talkspurts. In each of the schemes mentioned above, voice is treated as circuit-switched traffic and data as packet-switched traffic. Voice communication is somewhat tolerant to loss of information but quite intolerant to substantial delay. Data communication, on the other hand, is quite intolerant to loss of information but more tolerant to delay.
In the aforementioned movable boundary scheme, described by K. Sriram, et al., voice and data traffic dynamically share the channel capacity on a wideband transmission to link. Multiplexing is done within synchronous time-division multiplexed (TDM) frames. Frame duration usually is fixed. Each frame is divided into a number of time slots of equal duration and chosen according to the voice bit rate and the frame duration. Voice is digitized and packetized so that one packet is generated per frame. Voice and data packets are of equal duration. A predetermined number of time slots in each frame is reserved for data transmission so that surges of voice traffic do not cause excessive delays for data traffic. The rest of the time slots are shared by voice and data traffic with voice traffic given priority over data traffic. The data traffic is packet switched and transmitted on the link through a buffer on a first-in, first-served basis. Because of the buffering, data traffic may be delayed but not dropped.
Voice traffic is characterized by talkspurts and silent periods for every speaker being served. Silent periods are detected, and no packets are transmitted when the speaker is silent. If there are fewer voice packets to be transmitted than the number of shared time slots, then the voice packets are all transmitted. Some data traffic may be left-over after the predetermined number of data time slots per frame are used. Such left-over data traffic is transmitted during any of the shared time slots not being used for voice traffic. During a sequence of frames, the boundary between voice traffic and data traffic moves. Thus the scheme is named the movable boundary scheme. Occasionally the number of voice packets generated in a frame may exceed the number of shared time slots. The excess voice packets are dropped rather than delaying them, as in a buffering operation.
In the previously mentioned burst switching scheme, described by E. F. Haselton, a burst may be either a voice burst or a data burst. Each burst occupies one time slot per frame for the duration of the burst. Every burst is independently switched. A header, provided within each burst, provides the required routing information for guiding the switching function to assure that the burst is routed toward its destination. The switch establishes a path between links only for the duration that the burst is being switched. Links between switches are either standard DS1 lines or some other standard transmission rate.
Multiplexing on the links is based on a frame structure. All of the frames are of the same duration, which is divided into a fixed number of time slots. Each time slot accommodates one byte of eight bits. Multiplexing is done within synchronous time division multiplexed frames that are divided into synchronous time slots of bytes. A synchronous time slot, i.e., one time slot per frame, is regarded as a channel. The frame duration equals the voice sampling interval so that there is one voice sample per channel per frame. Voice is encoded at sixty-four kbps rate using pulse code modulation (PCM) and one byte per sample. When a voice burst arrives at a switch, the switch allocates a channel to that voice burst if a channel is available on the outgoing link toward the destination. Otherwise the burst waits for up to two milliseconds and then bits begin to be dropped from the front end of the burst. The clipping continues until an outgoing channel becomes available. A data burst, on the other hand, cannot be clipped. Hence it is stored at the input of the switch until a channel becomes available. Voice bursts have non-preemptive priority over data bursts for any outgoing channel allocation. Priority control messages have preemptive priority over data. Non-priority control messages have the same priority as data bursts.
The two schemes described above for integrated voice and data networks have certain disadvantages, or problems. In the movable boundary scheme, voice and data packets are required to be of the same duration. This is inefficient when transmitted packets are not fully populated with information. In this movable boundary scheme data has a number of reserved time slots; however, when the data traffic intensity is low, the idle time slots reserved for data go unused because voice traffic is not allowed in them. This is a problem because it lowers the efficient use of transmission capacity. In the burst switching scheme, long voice bursts can be readily integrated with relatively short data bursts; however, the scheme requires circuit-switched service for voice traffic and packet-switched service for data traffic. Voice bursts move quickly through the network whereas data bursts are queued and serviced via store-and-forward techniques. This hybrid switching requirement is a problem because it complicates the design of the switching node. Another disadvantage of the burst switching scheme is that it does not support bandwidth on demand by the user. Such flexibility in bandwidth allocation is not possible because transmission links are divided into channels of equal bandwidth via synchronous time slots. Each data burst is allocated to an individual channel. As a result intolerably long transmission delays occur for long data bursts.